My grapefruit tree has a bit of a problem; it appears to have an attack
of sooty mold
(http://aggie-horticulture.tamu.edu/C...htm#sootymold). On
fairly close inspection, there don't appear to be any insects on the
tree that would excrete the honeydew the mold feeds on.

So, two questions;
If it IS sooty mold, what else might be causing it?
If it ISN'T sooty mold, then what on earth is it???